High speed fiber-optic communication systems rely on advanced digital signal processing techniques to combat various fiber transmission impairments. One of the building blocks inside a digital receiver is a timing recovery circuit. The purpose of this block is to detect sampling frequency offsets and continuously restore correct timing for incoming digital samples. The establishment of digital synchronization is important for subsequent channel equalization and carrier recovery.
Timing error detectors (TED) based on Gardner's algorithm (F. M. Gardner, “A BPSK/QPSK Timing-Error Detector for Sampled Receivers”, IEEE Transactions on Communications, v. COM-34, p. 423-429, May 1986, the content of which is incorporated herein by reference) can be used to operate RF binary phase shift keying (BPSK) and QPSK digital systems. However, for fiber-optic channels, polarization and associated dispersion impairments are problems that need to be addressed carefully. For instance, a Gardner-based TED can lose detection sensitivity under certain polarization conditions, and thus can lead to malfunctioning of timing recovery circuits. (See, e.g., F. N. Hauske et al., “Impact of Optical Channel Distortions to Digital Timing Recovery in Digital Coherent Transmission Systems”, 12th International Conference on Transparent Optical Networks, p. 1-4, IEEE 2010, the content of which is incorporated herein by reference). Specifically, half-symbol differential group delay (DGD) together with equal mixing between the two polarization fields is recognized as a “dead zone” for conventional Gardner-based TED. Due to the dynamic nature of polarization in the fiber channel, this “dead zone” translates into unpredictable system outages which need to be solved before an optical transponder can be deployed in field links. Furthermore, this dead zone also prevents implementation of a transmitter side symbol interleaving scheme.
A proposed technique to mitigate this problem has been proposed by Sun. (Han Sun et al., “A Novel Dispersion and PMD Tolerant Clock Phase Detector for Coherent Transmission Systems”, OFC 2011, OMU4, the content of which is incorporated herein by reference). However, Sun relies on a secondary adaptation loop which needs to track two individual phase parameters. Furthermore, Sun only addresses the polarization rotation issue at a fixed half-symbol DGD. TED across the whole DGD range has not been demonstrated by Sun.
What is needed is polarization and differential-group-delay insensitive digital timing error detection for polarization-multiplexed coherent optical systems.